Generally, an image sensor is a semiconductor device that transforms an optical image to an electric signal. An image sensor can be classified as a charge coupled device (CCD) or a complementary metal oxide silicon (CMOS) image sensor.
The CCD has shortcomings such as a complicated driving method and high power consumption. Also, the fabricating method of the CCD is complicated because a multi-level photo process is required.
Therefore, the CMOS image sensor has received attention as a next-generation image sensor to overcome the shortcomings of the CCD.
The CMOS image sensor is a device employing a switching mode that sequentially detects outputs of each unit pixel using MOS transistors. The switching mode can incorporate MOS transistors formed on a semiconductor substrate in a number at least as many as the number of unit pixels, and can use a control circuit and a signal processing circuit as peripheral circuits.
The CMOS image sensor can be classified into types such as a 3T type, a 4T type, and a 5T type, according the number of transistors. For example, the 3T type CMOS image sensor includes one photodiode and three transistors, and the 4T type CMOS image sensor includes one photodiode and four transistors.
Hereinafter, the unit pixel of the 4T type CMOS image sensor will be described with reference to a plan view thereof.
As shown in FIG. 1, the unit pixel of the CMOS image sensor according to the prior art includes a photodiode 10 and four transistors as an optoelectric converter. The four transistors are a transfer transistor 20, a reset transistor 30, an access transistor 40 and a select transistor 50. In FIG. 1, FD denotes a floating diffusion region and a numeral reference 90 denotes a coupling portion connecting the FD and the access transistor 40. Vin denotes an input terminal, and Vout denotes an output terminal.
Hereinafter, the photodiode 10 and the transfer transistor 20 in the CMOS image sensor according to the prior art will be described with reference to a cross-section view through line I-I′.
As shown in FIG. 2, the transfer transistor 20 includes a gate insulating layer 21 and a gate electrode 23 formed on a substrate 11, and a first sidewall 29 and a second sidewall 31 formed at both sides of the gate electrode 23.
In addition an n− type diffusion region (N−) 28 and a Po type diffusion region (PDP; P type photodiode implant) 35 are formed at the photodiode region (PD) of the substrate 11. The Po type diffusion region 35 is formed on the n− type diffusion region 28. Also, a heavily doped n+ type diffusion region (N+) 32 and a lightly doped n− type diffusion region (N−) 26 are formed at the floating diffusion region (FD) of the substrate 11.
FIG. 3 is a cross-sectional view for describing one of processes for fabricating a CMOS image sensor according to the prior art.
As shown in FIG. 3, a photoresist layer 34 can be applied on an entire surface of the substrate 11, and the applied photoresist layer 27 can be patterned to expose the photodiode region (PD) through an exposing and developing process.
Then, a Po type diffusion region 35 can be formed in the n− type diffusion region 28 of the photodiode region by implanting Po type impurity ions using the patterned photoresist layer 34 as a hard mask.
However, the method for fabricating a CMOS image sensor according to the prior art has problems as follows.
The relevant art's fabricating method becomes complicated because a photolithography process, which includes applying a photoresist layer, exposing and developing the photoresist layer, implanting impurity ions, and removing the photoresist layer, is performed in order to form the Po type diffusion region (PDP) at the photodiode region.